The Role of Sex Hormones in Mammalian Reproduction The male sex hormone is testosterone and the main female sex hormone is estradiol which belongs to a class of hormones called estrogens Testosterone is synthesized in specialized cells inside the testes Estrogens are produced in the ovaries by cells that surround the developing egg which form a structure called a follicle 2011 Pearson Education Inc The Role of Sex Hormones in Mammalian Reproduction Human sex hormones play a key role in 1 Development of the reproductive tract in embryos 2 Maturation of the reproductive tract during the transition from childhood to adulthood 3 Regulation of spermatogenesis and oogenesis in adults 2011 Pearson Education Inc Which Hormones Control Puberty in Mammals A number of changes take place in boys and girls during puberty the process that leads to sexual maturation in humans In amphibians the juvenile to adult transition is triggered by the hormone T3 triiodothyronine In insects the transition to adulthood occurs in response to ecdysone These changes are triggered by increased levels of testosterone in boys and estradiol in girls 2011 Pearson Education Inc 2011 Pearson Education Inc What Regulates the Gonadal Hormones Puberty is initiated when gonadotropin releasing hormone GnRH is released from the hypothalamus This triggers release of luteinizing hormone LH and folliclestimulating hormone FSH from the pituitary gland LH and FSH then trigger increases in testosterone and estradiol In adolescents experiencing delays in the onset of puberty GnRH treatment induced surges in LH and FSH followed by puberty onset 2011 Pearson Education Inc 2011 Pearson Education Inc What Regulates the Regulatory Hormones There is some evidence that nutritional state is involved in triggering GnRH increases at the appropriate age Puberty onset in the United States has dropped from age 17 during the eighteenth and nineteenth centuries when the general nutritional state of the population was poorer to age 12 today Among girls living today individuals with large fat stores tend to enter puberty earlier than do girls who are thin 2011 Pearson Education Inc Hormones Control the Mammalian Menstrual Cycle The menstrual cycle is a monthly reproductive cycle that occurs in the ovary and uterus It averages 28 days in humans In conjunction with changes in the ovary the uterine lining undergoes a dramatic thickening and regression and a part of the lining ultimately sloughs off The onset of the combined ovarian and uterine cycles is the start of menstruation the expulsion of the uterine lining through the vagina This event is designated as day 0 in the cycle 2011 Pearson Education Inc Hormones Control the Mammalian Menstrual Cycle Only humans and other great apes menstruate Female mammals that do not menstruate have what is called an estrous cycle The menstrual cycle has two phases 1 In the follicular phase which lasts an average of 14 days the follicle matures and ovulation occurs 2 During the luteal phase which also lasts an average of 14 days a structure called a corpus luteum forms from the ruptured follicle and subsequently degenerates 2011 Pearson Education Inc 2011 Pearson Education Inc Hormones Change during a Menstrual Cycle By monitoring hormone concentrations in the blood or urine of a large number of women over the course of the menstrual cycle researchers were able to document dramatic changes in the concentrations of estradiol and several other hormones During each cycle LH and FSH are produced in the anterior pituitary gland in response to GnRH The steroid hormone progesterone is produced along with estrogens including estradiol in the ovaries 2011 Pearson Education Inc Hormones Change during a Menstrual Cycle LH stays fairly constant except for a spike that begins just before ovulation suggesting LH might be the trigger for this event FSH levels are relatively high during the follicular phase and low during the luteal phase Progesterone is present at very low levels during the follicular phase but at high levels during the luteal phase Estradiol surges during the follicular phase while progesterone surges during the luteal phase 2011 Pearson Education Inc 2011 Pearson Education Inc How Do Pituitary and Ovarian Hormones Interact Changes in the concentration of estradiol and progesterone affect the release of the pituitary hormones LH and FSH Estradiol exerts negative feedback on LH and FSH at low levels but positive feedback at high levels In other words estradiol s effect on the pituitary hormones is dosage dependent Progesterone exerts only negative feedback on the two pituitary hormones 2011 Pearson Education Inc Day 0 7 of the Menstrual Cycle As the uterus is shedding much of its lining a follicle is beginning to develop in one ovary under the influence of FSH The follicle produces estradiol and a small amount of progesterone While its levels are still relatively low estradiol suppresses LH secretion through negative feedback inhibition 2011 Pearson Education Inc Day 8 14 of the Menstrual Cycle As the follicle grows its production of estradiol gradually increases The increase in estradiol stimulates mitosis and an increase in cell number in the uterine lining The enlarged follicle produces large quantities of estradiol which begin to exert positive feedback on LH secretion Positive feedback results in a spike in LH levels just after estradiol concentrations peak The LH surge triggers ovulation and ends the follicular phase 2011 Pearson Education Inc Day 15 21 of the Menstrual Cycle As the corpus luteum develops from the remains of the ruptured follicle it secretes large amounts of progesterone and small quantities of estradiol in response to LH The rise in progesterone lowers production of LH and FSH and activates the thickened uterine lining creating a spongy tissue with a well developed blood supply In this way progesterone fosters an environment that supports embryonic development if fertilization occurs 2011 Pearson Education Inc Day 22 28 of the Menstrual Cycle If fertilization does not occur the corpus luteum degenerates Progesterone levels fall as the corpus luteum shrinks When progesterone declines the thickened lining of the uterus degenerates GnRH LH and FSH are released from the inhibitory control that progesterone exerts LH and FSH levels rise and a new menstrual cycle begins 2011 Pearson Education Inc 2011 Pearson Education Inc